After the match when the control system is disabled, will pneumatic cylinders hold thier position still? Sorry for the newbie question but my team is very experienced in everything but pneumatics.
They will hold their position assuming you still have pressure in your accumulators, at least thats what Ive always found the case to be when we’ve used them.
Cory
Whether or not the pneumatics will hold their position after a match depends on how the actuators are connected. If they are controlled by a double solonoid valve, they WILL hold their final position after the match ends. (provided there are no significant air leaks).
Cylinders controlled by single solenoid valves will return to the default “off” position when the match ends. (note that the default off position might mean extended or retracted, depending on how you connect the tubing).
The double solenoid valves will cause a pneumatic actuator to hold their position after a match has ended, but single solenoid valves might not. Single solenoid valves have a default position that they automatically (because of a spring, usually) revert to when power to the acutal solenoid is cut. If the actuator is in the non-default position at the end of the match, and power is cut, the valve will switch Pressure and Exhaust ports, and the actuator will change states.
Basically, the way to make an actuator hold its position after the match ends is to use a double solenoid valve.
Hope this helps. 
<edit>Ack, a little late. I’ll leave it up, anyway.</edit>
one caviot: they will only hold as described above if your system has no leaks
Ive seen bots with arms extended at the end that sloooooooowwllly drooped while the judges were scoring - if your pnuematics leak, the final resting position is what counts.
If you want something to stay put at the end, put a latching mechanism on it - something that will have to be manually released to retract.
Well it’s always best to test you connections to make sure there are no leaks prior to putting it on your robot, it will be easier to find a a leak if its sprawled out over a table.
Definetly true kevin.
Building a pneumatics board to practice on makes it so much easier to put it in the robot.
Their are many ways to leave a actuator extended, retracted, or even somewhere in the middle after the power has been cut. The best way is to read the pneumatic manual on FIRST and play with the different parts. Remember the single solenoid switches back, the double stays on one side, and that air is a fluid.
Pneumatics are fun.
Oh yeah, to check for leaks get water with a little soap in it and check each part. Or just have really good ears in a quiet environment, sssssss, not best to look for in the pits or before competition.
Attached is how a Single Solenoid works, the Magnet symbol is for the ON and “Not ON” states. Top is ON, bottom is Not ON. The Orange color is exaust route, light blue is pressured route. To the direct right of the magnet symbol is the spring in it’s compressed and extended states.
Note: all 5 positions in the diagram have plugs and wires. The bottom left and bottom right do not require a plug or wire if you do not intend on using the exhaust.
Yes, using a dubble solenoid valve will keep it’s poition after the power has been cut at the end of the match. However, the pressure regulators relieve excess air pressure in the system. On page 4 of the penumatics manual, It describes this process as:
“Assume that you extend the cylinder of the apparatus the cylinder is attached to agaainst a wall. Then push against the wall with your robot. That would increase the preasure in the cylinder. The increased pressure will relieve out of the regulator and the cylinder will slowly retract.”
Now, immagine your robot as a weight the the cylinder has either extended or retracted, which will cause a greater preassure than 60psi (if your robot weight under the cylinder is greatter than 45 pounds) on a 3/4" bore cylinder or if the robot is greater than 90 pounds under the cylinder on a 1.5" bore. In order for you to get around this problem, use a 2" bore with a robot weight under 120 pounds.
Take note that this applies when you exert force greater than the capacity of 60PSI over the given area.
For example:
Robot A using a 2" Bore cylinder retracts the cylinder to pull iteself into a HANGING position.
Robot B is using a Winch-Type mechanism and HANGs higher than Robot A.
When power is cut, Robot A will stay in position. For examples sake, let’s say Robot B’s winch slowly slips and it’s frame begins to be supported by robot a:
-----X----X------
| |
| |
|[ROBOT_B]
[ROBOT_A]
If Robot B’s weight being rested on Robot A is more than ~40 lbs, Robot A will start to slowly sink down towards the ground, if it’s only support is the Cylinder.
If you have a lot of time to test, a “5-Minute” or “10-Minute” test is a great leak checking measure. Charge the pneumatics assembly to 120psi, turn off the power, and check the guages 5 or 10 minutes later. If they still read 120, you have no leaks. If thee gauges are under 100, you have a slow leak, if the guages are under 60 psi, you have a problematic leak.
If it’s your first time using pneumatics, or first time using the soap method, take about 2 oz of water and maybe a 1/2 oz. of liquid soap mixed well. Should be noticible to the touch that there is soap present, but just soap won’t work well. Using a finger, dip it into the soap/water mixture, then dab around any place where a connection is made. Plugs to cylinders, plugs to tubes, plugs to T-Box’s… anywhere that a leak could be. You will notice that the air pushing out of the leak will make little, but noticeable, bubbles. tighten the connection or recut the hose to fix the leak. (Note: messy cuts in hoses are usually the cause of the problem. If it doesn’t cut the first time, go 1/4" higher on the tube and cut again. Tubing is rather lightweight. An extra 1-3 inches per plug is a good saftey measure incase tubes need to be cut down.
When you do your calculations, remember that the cylinder has different power capabilities in each direction. Pushing the rod out has more force because there is 60 psi acting over a larger surface area. On the retract, the surface area is reduced by the diameter of the rod, so you have 60 psi acting on a smaller surface area. Depending on the specific cylinder, the difference in cabability can be substantial.
There is leak locator you can buy which is basically liquid soap. Ours came with a giant Q-tip attached to the cap.
The soap and water thing works great, but set it up on a board to test it, and test it ALOT, ALOT, ALOT!!!
Pnuematics barring leaks and other snaffoos (sighs at two years ago’s robot that leaks like a spaghetti strainer) should hold thier position as long as they have the particular valve (forget which one it is) two have a spring return and the 3rd uses air power to turn on and air power to return that would be the one you’d want to get if your objective is to stay after time is called.
If you use the double solenoid valve it will hold the cylinders position when power is turned off.
a double solonoid valve will hold its position.
A single solonoid will shift to its rest position – so it either will hold its position if it starts in rest position or it will shift(and the cylinder its attached to will also shift) if you have it in activated position when the match ends.
BE CAREFUL WHEN TIGTENING
we had a bit of a snafu lat year with the port where the 125 psi presure release valve was connected leaking last year, so my associate wipped out the 12" long wrench and tightened it, cracking the port on the compressor, and breaking the valve. that compressor is now irriparible…
The easiest way to have a pneumatic cylinder stay in position after your robot has it’s power turned off, is to use the double selenoid valve. If you notice that you have a leak in your system, the easiest way i found to find a leak, is to take a cup of soapy water with a painting brush and go over all the connectors in the system. Turn the system on, if you see a bubble appear, you have a leak in that particular valve. Take the connection off, put some more teflon tape, tighten the connection enough, and re-check. We had this problem last year and this solution solved everything.
I guess a mechanical solution would be the chinese finger trap idea, that maybe after the cylender extends, it extends passed a spring loaded switch. like how classroom doors are locked, so you cant turn the knob on the outside, but the latch can retract when you close the door. that would be the faisafe, but if you were off, you might bend the cylender, or it might get stuck. I’d stick with the double solenoid and leak prevention though.